Make getData() more flexible for remote and local images

Make getData() more flexible, by allowing any object with a src property to be passed to it. This is useful to avoid unnecessary requests triggered if you set src on an Image object, so you can directly load a remote image without having to create such an object, e.g.

```
EXIF.getData({src: 'http://my.image.url'}, function () { ... });
```

I also modified the condition which checks whether the Ajax request completed correctly, so that it doesn't rely on status 200 (it is 0 for local files).